Magneto-optic memory medium

ABSTRACT

A magneto-optic memory medium is disclosed which is made up by an acryl resin substrate having strip-like grooves, a magneto-optic magnetization film disposed on the substrate and having an axis of easy magnetization perpendicular to its surface. The magnetization film is made of amorphous ferromagnetic material such as GdTbFe, GdTbDyFe, SmTbFe, GdDyFe, TbCo or the like. A reflective film is provided overlying the magneto-optic magnetization film.

BACKGROUND OF THE INVENTION

This invention relates to a magneto-optic memory medium with whichwriting, reading and erasing of information are performed throughapplication of a laser beam.

In recent years, a substantial amount of effort has been directed towardthe development of an optical memory medium which satisfies variousrequirements including high density, large capacity and high speedaccess.

Of a wide range of various optical memory media magneto-optic memoryelements comprising a perpendicular magnetization layer as a memorymaterial are most attractive thanks to its unique advantage thatinformation can be erased after use and new information can be writtenthereon. The magneto-optic memory elements however have the disadvantagethat they provide weak reproduced signals and exhibit badsignal-to-noise ratio (S/N ratio). Especially when reproduction ofinformation relying upon reflected light from the magneto-opticelements, that is, the Kerr effect reproduction system is in use, theKerr rotation angle of magnetic material is small and an improvement inS/N ratio is almost impossible. For this reason the prior art approacheswere made in an attempt to increase the Kerr rotation angle; animprovement in magnetic material as a recording medium; and depositionof such a dielectric film as SiO and SiO₂. It was reported in"J.Appl.Phys." Vol. 45, No. 8, August 1974, for example, that the Kerrrotation angle increased from 0.7° to 3.6° by deposition of a SiOcoating on a MnBi magnetic thin film as an example of the latter.However, the deposition of such a dielectric coating on a magnetic filmincreases the Kerr rotation angle, but on the other hand decreases theamount of the reflected light and S/N ratio improved no more than abouttwice. An improvement in S/N ratio is, therefore, not expected to atheoretical extent. In the case that the dielectric thin film typicallyof SiO and SiO₂ is deposited, it can not protect the magnetic materialagainst corrosion. It is also impossible to sense recording bits whenthe diameter of the recording bits is of the order of 1 um and dust orother foreign objects of a diameter of approximately 1 um. It istherefore desirable that the thickness of the memory elements be between0.5 and 2 mm for practical application. However, this requirement is inconflict with the above discussed requirement of increasing the Kerrrotation angle.

Meanwhile, an approach has been suggested; an amorphous magnetic filmsuch as DyFe is deposited on a garnet substrate and information recordedon the DyFe film is transferred to the garnet substrate of good S/Nratio for readout (e.g., "Digest of the Fourth Annual Conference onMagnetics in Japan", 5aB-4). However, this method does not appear to besuitable for large capacity memory application because of difficulty inmaking a large area memory medium.

Apart from the foregoing problems, high density recording isindispensable for optical memory elements. Accordingly, since thediameter of the recording bits is of the order of 1 um as statedpreviously, survo technique including a focus survomechanism and a tracksurvomechanism is necessary during writing, reading and erasing.Otherwise, there is the need for a recording mechanism which is toocomplex and precise for practical application. Unlike the Phillips videodisk equipment of the MCA type which requires only reproducinginformation previously recorded, the magneto-optic recording deviceshould write new information at the position where no information iscontained, when the track survomechanism is in use. It is thereforedesirable that guide tracks be formed in parallelism withsignal-recording tracks for helping the survomechanism.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amagneto-optic memory medium where guide tracks are available for asurvomechanism with magneto-optic enhancement without decreasing theamount of reflected light.

According to a broadest aspect of the present invention, a magneto-opticmemory medium comprises a substrate having strip-like grooves, amagneto-optic magnetization film disposed on said substrate and havingan axis of easy magnetization perpendicular to its surface, and areflective film overlying said magneto-optic magnetization film.

In a preferred form of the magneto-optic memory medium, there isdeposited on an acryl resin substrate a magnetic TbDyFe film having anaxis of easy magnetization perpendicular to its surface. The acryl resinsubstrate is highly resistive to shock and easy to handle. Furthermore,guide tracks may be formed in the acryl resin more easily. The TbDyFefilm has a Curie point of 75° C. and the acryl resin substrate has asofting point of less than 85° C., so that the acryl resin substrate iscapable of undergoing extended use.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther objects and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross sectional side view, partially in an enlarged scale,showing a magneto-optic memory medium according to an embodiment of thepresent invention; and

FIG. 2 is a cross sectional side view, partially in an enlarged scale,showing an exemplary substrate.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated in cross sectional sideview a magneto-optic memory medium according to an embodiment of thepresent invention. A substrate 1 typically of acryl resin or glass withstrip-like grooves 7 is prepared by a well known manner. In the case ofthe substrate 1 of acryl resin, plastic molding is available by which toform a grooved substrate at one time. Deposited on the substrate 1 is aperpendicular magnetization layer 2 of amorphous ferrimagnetic materialconsisting of rare earth metal and transition metal, for example,GdTbFe, GdTbDyFe, SmTbFe, GdDyFe, TbCo, etc., by sputtering, evaporationor other well known methods. The amorphous layer 2 is overlaid with atransparent dielectric layer 3 typically of SiO₂, SiO, MgF, TiO₂ or thelike which in turn is overlaid with a reflective layer 4 made of Al, Cu,Au, Ag, Zn, Sn or the like. Furthermore, the reflective layer 4 liessandwiched between the dielectric layer 3 and a support plate 6 by meansof a proper adhesive. With such a medium structure, recording,reproducing and easing information are performed via the substrate 1.Recording tracks may be formed either in the strip-like grooved portionsor valley portions 7 or non-grooved portions or ridge portions 8. Thisis true with guide tracks. In other words, the guide tracks may beformed fully in either the valley portion 7 or the ridge portions orhalf in the valley portions 7 and half in the ridge portions 8. Signalsare therefore available for a track servo system through a differentiallevel in the substrate 1. In the case where the magneto-optic medium isof a disc configuration, the strip-like grooves 7 may be formedcoaxially or spirally. The magneto-optic film 2 is so sufficiently thinthat reproducing light incident upon the magnetization layer undergoes acombination of the Kerr effect caused by reflected light from thesurface of the magnetization layer and the Faraday effect caused bylight penetrating the magnetization layer, reflecting on the reflectivelayer 4 and traversing again the magnetization layer and increases itsrevolution angle several times as much as that caused by merely the Kerreffect. The amount of returning light does not show less or no decrease.

The gist of the present invention rests in a combination of the groovedsubstrate and the reflective layer for enriching the magneto-opticperformance, so that many modifications and changes are possible withinthe spirit of the invention. For example, the dielectric layer 3 may beeliminated or a metallic thin film of Al, Cr, Ti or the like may beprovided between the dielectric layer 3 and the magnetization layer 3. Aunit consisting of the substrate 1, the magnetization layer 2, thedielectric layer 3 and the reflective layer 4 may be bonded by the useof an adhesive 5 without using the support plate 6, thus realizing aboth-sided magneto-optic memory medium. Further, as shown in FIG. 2, thesubstrate 1 may be set up by a lamination of a grooved plastic substrate10 and a glass substrate 9. The recording tracks and the guide tracksare not necessarily required to be strip-like. Informationcharacteristic of track identifying numbers and sector identifyingnumbers may be written.

The grooved-substrate 1 offers the advantage over the flat substrate:easiness of manufacture and simplicity of a manufacturing equipment,because the grooves in the substrate are helpful in positioning themanufacturing machine. On the contrary, the flat substrate requirescomplex and large-sized machining relying upon an air shaft.

As stated previously, the substrate is typically made of acryl resin andthe magnetization layer is of TbDyFe with an axis of easy magnetizationperpendicular to its surface. The acryl resin substrate is highlyresistive to shock and easy to handle. Furthermore, guide tracks may beformed in the acryl resin more easily. The TbDyFe film has a Curie pointof 75° C. and the acryl resin substrate has a softing point of less than85° C., so that the acryl resin is capable of undergoing extended use.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications are intended to be included within the scope of thefollowing claims.

What is claimed is:
 1. A magneto-optic memory medium comprising:asubstrate having strip-like grooves, a magneto-optic magnetization filmdisposed on said substrate and having an axis of easy magnetizationperpendicular to its surface; and a reflective film overlying saidmagneto-optic magnetization film.
 2. A magneto-optic memory mediumcomprising:a substrate having strip-like grooves and made of an acrylresin, said grooves being formed during molding of said substrate; amagneto-optic magnetization film disposed on said substrate and havingan axis of easy magnetization perpendicular to its surface; and areflective film overlying said magneto-optic magnetization film.
 3. Amagneto-optic memory medium as set forth in claim 1 wherein said groovesare used for the formation of guide tracks.
 4. A magneto-optic memorymedium as set forth in claim 1 wherein said magnetization film is madeof an amorphous ferrimagnetic material.
 5. The magneto-optic memorymedium as set forth in claim 4 wherein said magnetization is made of aferrimagnetic material, which includes a rare earth metal and atransition metal.
 6. The magneto-optic memory medium as set forth inclaim 1 wherein said reflective film is made of a member selected fromthe group consisting of Al, Cu, Au, Zn and Sn.
 7. The magneto-opticmemory medium as set forth in claim 5 wherein said magnetization film ismade of TbDyFe.